Switching waveforms of power converters generate electromagnetic interference (EMI) at the fundamental switching frequency and its harmonics. The EMI (i.e., noise) is transmitted in two forms: radiated and conducted. Usually conducted noise is several orders of magnitude higher than the radiated noise in free space. The conducted noise consists of two categories commonly known as the differential mode and the common mode. The differential-mode noise is a current or voltage noise measured between the lines of the source. The common-mode noise is a current or voltage noise measured between the power lines and the ground. Both differential-mode and common-mode noises are taken into account in EMI filter design, with common-mode noise being the dominant factor.
Passive EMI filters employ only passive components and provide a rudimentary level of EMI filtering. Passive filtering technique is simple and cost effective in some applications; however, in applications where stringent noise reduction is required, the size, weight, effect of temperature, and reliability can present a significant design challenge.
Active analog EMI filters use active analog components such as op-amps to reduce EMI noise in circuits such as power converters. This technique samples the noise signal, processes it, and injects it back into the circuit to cancel the incident noise signal. The EMI noise is reduced by superposition of two signals with the same magnitude and opposite phase propagating in the same direction. Such active analog EMI filters provide better noise suppression over passive filters, are low cost, and are generally convenient to implement. At high frequencies the performance of analog active EMI filters is limited by the frequency characteristics of the active device. Hence, such a filter requires additional passive elements to complete the electromagnetic compatibility (EMC) spectrum. At the same time, any negative impedance seen by the power converter, due to passive components of the EMI filter and the filter implementation, can have a negative impact on the power converter stability. In addition, analog filters may be bulky as their size is product-specific and varies with the input parameters of the power converter, such as rated current and voltage.